Const fin='litere.in';
      fout='litere.out';
Var
   f:text;
   Ap:Array['a'..'z'] of integer;
   k,i,n:integer;
   c,ch:char;
Begin
     assign(f,fin);
     reset(f);
     readln(f,n);
     for i:=1 to n do
     begin
         Read(f,c);
         Inc(Ap[c]);
         for ch:=succ(c) to 'z' do
             k:=k+Ap[ch];
     end;
     close(f);
     assign(f,fout);
     rewrite(f);
     Write(f,k);
     Close(f);
End.